It is hypothesized that it is possible to image in practical times the spatial distribution of photon emitters in the 3-10 keV range with 10-1_m resolution over a field of view of about 5 mm with a CCD camera and coded aperture optics. The specific aim of the proposed research is to build and test a soft X-ray camera of novel design based on coded apertures. The target application is imaging of characteristic X-rays, such as from K lines of elements from P to Ge, (including notably K, Ca, Cr, Mn, Fe, Cu and Zn) and L lines of heavier elements, from Ag to Hg (including I and Au). The significance of the project resides in its potential to image dynamically the distribution of biologically significant isotopes with an unprecedented resolution of 10 _tm, i.e. on the same space scale as individual cells. Diffusion mechanisms of electrolytes and metabolites, the biochemistry of elements such as K, Ca, Fe, Cu and I, and cell migration, signaling and trafficking are all examples of fundamental cell biology problems to which the technique could be applied. The limited penetration of these photons (tenths of a millimeter) suggests mainly in vitro applications of the technique, but superficial in vivo uses may also be possible. Coded aperture imaging uses thousands of pinholes arranged in structured patterns to overcome the well-known limitations of pinhole imaging on sensitivity, field of view and distance from the object. However, due to multiplexing, the sensitivity increase does not always translate in a signal-to-noise ratio (SNR) increase. The SNR advantage is clear in the case of concentrated distributions of the activity over the field of view, such as may be expected in the sample biological problems mentioned above, for which a sensitivity increase as high as 4 to 5 orders of magnitude can fully translate in increased SNR. Coded aperture optics will be coupled to a CCD camera, which provides an efficient, convenient and available means of imaging photons in the energy range of interest. The coded aperture will be designed applying the same geometrical considerations recently used in the design of small-animal imaging systems for 140 keV photons. Performance will be first simulated with computer codes and then experimentally verified in the case of different test sources designed to simulate biologically relevant cases.